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Homework Help: 3d vectors question

  1. Oct 1, 2014 #1
    1. The problem statement, all variables and given/known data
    A man is standing in his home , his position his facing direction and the position of the sweet dish are given ,if he faces the sweet dish , he eats it .
    Translating it into CS programming question : write a function ->
    bool IsManInFrontofDish( vector3& manPosition, vector3& manFacingPos, vector3& dishPos )
    which return true when the man faces the dish;

    The programming part is easy , I was worried about the physics part .
    Is the manPosition really important in this , or is it just to deceive?
    I am attempting a physics programming question for the first time...
    2. Relevant equations
    A.B = ||A||.||B||

    3. The attempt at a solution
    the angle between 2 vectors is 0 if they are in the same direction so simply by replacing manFacingPos with A and
    dishPos with B and checking the equation i will be able to get the answer:
    translating it into program if it helps :

    bool IsManInFrontofDish( vector3& manPosition, vector3& manFacingPos, vector3& dishPos ) {
    int magnitude1 = sqrt( dot(manFacingPos, manFacingPos) ); //square root of dot product
    int magnitude2 = sqrt( dot(dishPos,dishPos) );
    if(dot(manFacingPos, dishPos) == magnitude1*magnitude2) return true;
    else return false;

    P.S. I am not attending any university or school right now , so this is not a homework question
  2. jcsd
  3. Oct 3, 2014 #2


    User Avatar

    Staff: Mentor

    It would probably be easier to take the two positions (man and sweet dish) and form a unit vector pointing from the man to the dish. Then form a unit vector from the man's facing direction vector. Compare them.

    That the man's position is important should be clear. Make a sketch. Keeping the same facing direction vector, move the man about the room and see if he ever faces the sweet dish depending upon his location.
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